1. Field of the Invention
The present invention relates to communications signal amplifiers and to power control and amplitude modulation control.
2. State of the Art
High linearity and high energy efficiency are two principal (and usually competing) objectives of conventional communications signal amplifier design. High linearity is required to produce “clean” communications signals that produce a minimum of interference and provide high quality of service. High energy efficiency is desirable particularly for portable, battery-powered devices, as well as for purposes of reducing infrastructure costs.
In a cellular telecommunications system, handsets and other devices communicating with the radio base station are required to transmit at one of many possible power levels depending on the proximity of a particular device to the base station. This average output power control function may for some standards entail a dynamic range of 80 dB. Achieving linearity over this wide dynamic range has proved problematic.
Referring to FIG. 1, a simplified diagram of a conventional communications signal transmitter is shown. A signal generator 101 generates a signal having both amplitude and phase modulation. This signal is applied to an amplitude varying circuit 103 such as a variable gain amplifier (VGA) or variable attenuator controlled in accordance with a power level signal 104. A resulting scaled signal is then applied to a linear amplifier 105 to achieve final amplification. In this arrangement, because the amplitude-modulated signal passes through the amplitude varying circuit 103, the amplitude varying circuit must have very linear performance.
An alternate approach involves polar modulation, in which separate amplitude and phase paths are provided. Polar modulation architectures (and similar architectures in which separate amplitude and phase paths are provided) are described, for example, in U.S. Pat. Nos. 6,191,653, 6,194,963, 6,078,628, 5,705,959, 6,101,224, 5,847,602, 6,043,707, and 3,900,823, as well as French patent publication FR 2768574, all of which are incorporated herein by reference.
Referring in particular to U.S. Pat. No. 6,191,653, an RF power amplifier architecture is described in which separate phase and amplitude paths are provided. The amplifier has multiple stages. For signals having a comparatively high average power level, amplitude modulation is achieved in a final amplifier stage operating in a non-linear mode. In the case of a FET (field effect transistor) active device, for example, drain modulation is applied to the FET in order to impress the desired amplitude modulation upon the output signal. For signals having a low average power level, amplitude modulation is achieved in a preceding amplifier stage (again using drain modulation), the final amplifier stage being operated in linear mode (see column 6). In this manner, a range of amplitude modulation is provided that is larger than can be supported by just drain modulation of the final stage. Achieving an 80 dB dynamic range of average output power using this arrangement, however, remains problematic. Furthermore, any non-linearities in amplitude modulation performed in the driver stage are magnified in the final stage.
The present invention addresses the need for wide dynamic range of average output power without requiring high linearity. When applied in conjunction with switch mode amplification techniques, overall high efficiency may be achieved.